Thranduil was one of the Sindar, or Grey Elves, the son of King Oropher. King Oropher lead the forces of the Greenwood against Sauron in the Battle of Dagorlad before the Black Gate of Mordor, in the Last Alliance of Elves and Men at the end of the Second Age. Oropher was slain in that battle and his son was later crowned king.

During the Third Age Thranduil's people migrated northward in the forest of Greenwood the Great, especially after the year 1000 when a shadow began to fall on its south. Thranduil's folk eventually settled in north-east Mirkwood, beyond the Forest River. Here they managed to hold the evil emanating from the feared fortress of Dol Guldur at bay and lived lives of guarded peace in both the forest and the great underground halls of Felegoth which they built.
Being located so far north, and on the eastern edge of an increasingly perilous Mirkwood, Thranduil's realm became somewhat isolated, but he established cordial diplomatic and trade relations with the Dwarves and Men who lived nearby in Erebor, Dale, and Esgaroth. However, the attack of the dragon Smaug in TA 2770 destroyed Erebor and Dale, and reduced Esgaroth to a shell of its former self; though there remained a healthy wine trade between the lake and the wood.
Thranduil did what he could to aid his allies during the War of the Ring, including holding the creature Gollum in his dungeons for interrogation by Gandalf on the history of the One Ring. Gollum soon escaped with the aid of Orcs, and Thranduil sent his son Legolas to Rivendell to seek the counsel of Elrond and Gandalf on this and other matters.
Thranduil and his people withstood attacks by Sauron's forces during northern battles of the war. Having routed their foes in the north, Thranduil's forces moved south, and joined with the armies of Lorien under Celeborn and Galadriel in destroying Dol Guldur, cleansing Mirkwood of Sauron's taint of evil. Thranduil, along with Celeborn, renamed Mirkwood Eryn Lasgalen, The Wood of Greenleaves. After the war, Thranduil's realm was expanded to include all Mirkwood north of its central mountains, and he and his people enjoyed peace.

Thranduil (The Last Alliance)

Prince Thranduil is encountered by High Elf characters in the year 3434 fighting with the elvish host of the The Last Alliance of Elves and Men during the battle at Dagorlad. He inherited the crown to the kingdom on his father's death in that battle.

Thranduil (Felegoth)

Now King, Thranduil is ruler of Eryn Lasgalen and is holding court in Felegoth. He will use what help he can to clear the last of the forces of Sauron from the woods and to re-establish the roads and trade with neighbouring peoples and kingdoms.